State of the art photo lithography processes for integrated circuit (IC) manufacturing require OPC to transfer with high fidelity intended patterns onto a photoresist, to correct for optical distortion due to the diffraction of light going through the photomask during the photolithography process. OPC is based on optical and resist models to predict with high accuracy the way photomask shapes print after resist development.
The resist model is highly empirical and therefore requires careful calibration by precise measurement of the dimensions of well-chosen structures including test structures and structures that are critical to the design. Traditionally, a measurement or critical dimension (CD) is obtained for each scanning electron microscopy (SEM) image, and an equivalent simulation cutline is set at the center of the layout (perpendicular to the layout) to be simulated. For example, as illustrated in FIG. 1A, in a CD based metrology, each structure 101 gives rise to a single measurement 103 per image. The measurement is targeted at a metrology window 105, and the equivalent simulation cutline 107 (shown in FIG. 1C) is set perpendicular to the layout to derive the image parameters.
Since the number of structures that can be measured is limited (typically less than 1000), one challenge is to select structures that best represent the diversity of all possible full chip designs, as empirical models are not suited for extrapolating too far from the calibration space. In addition, another challenge is to minimize the redundancy within the selection to avoid overweighting specific structures and biasing the OPC model, thereby reducing the model's capacity to predict. These challenges are greatly increased when using SEM contour based metrology to calibrate OPC models, as illustrated in FIG. 1B, as each structure 109 supplies hundreds of measurements or sites per image, depending on the contour sampling (for example, sampling with a value of 1 to 10 nanometers (nm) which leads to hundreds of corresponding cutlines 111 perpendicular to the contours (187 cutlines shown in FIG. 1D), to derive the image parameters. Using SEM contours to calibrate the OPC model, it is common to deal with over 10,000 measurements for the calibration. Since each image gives more than one measurement, the ability to optimize the structure selection (inter-structure selection) is insufficient. In addition, selection of measurements within the selected structures of the image (intra-structure selection) must also be optimized.
Traditionally, the position of the simulation cutline to derive image parameters is centered at the edge of the layout to be simulated, where the layout can be the intended drawn layer or the photomask layer. However, the position of the cutline can then differ significantly from the resist edge. Furthermore, the accuracy of the measurement can hardly be promising when the layout is made of polygons with sharp corners that translate into round shapes in the resist. Thus, the simulation cutlines close to a corner tend not to be perpendicular to the resist edge, and the derived image parameters will not be accurate. A need therefore exists for increasing the accuracy of the measurements of each targeted structure, and optimizing the selection of the structure, to represent the shape of the photomask, for OPC model calibration.